Gasoline production from wellhead natural gas

ABSTRACT

A PROCESS FOR THE PRODUCTION OF GASOLINE FROM WELLHEAD NATURAL GAS IS DISCLOSED IN WHICH LIGHT HYDROCARBON COMPOUNDS FOUND IN WELLHEAD NATURAL GAS ARE CONVERTED BY AN INTEGRATED PROCESS OF OPERATIONS INTO SALABLE GASOLINE AND NATURAL GAS PRINCIPALLY COMPOSED OF METHANE.

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GASOLINE PRODUCT GEORGE W. ERICK INVENTOR.

BY WW ATTORNEY.

United States Patent 3,660,272 GASOLINE PRODUCTION FROM WELLHEAD NATURALGAS George W. Frick, I-Iightstown, N.J., assignor to Cities Service OilCompany, Tulsa, Okla. Filed Dec. 30, 1970, Ser. No. 102,687 Int. Cl. Cg39/00 US. Cl. 208-93 5 Claims ABSTRACT OF THE DISCLOSURE A process forthe production of gasoline from wellhead natural gas is disclosed inwhich light hydrocarbon compounds found in wellhead natural gas areconverted by an integrated process of operations into salable gasolineand natural gas principally composed of methane.

BACKGROUND OF THE INVENTION The present invention relates to theproduction of gasoline from wellhead natural gas. More particularly, theprocess of the present invention is for the production of gasolinesuitable for use as motor fuel from compounds found in wellhead naturalgas.

Usually, the low boiling range of light hydrocarbon intermediates,including ethane, propane, normal butane, isobutane and natural gasolineprecludes or limits the use of these components directly as motor fuel.Ethane, propane, and butane tend to be used as petrochemical feedstocksor as a source of household bottled gas. In addition, the octane qualityof natural gasoline, the C and higher boiling hydrocarbons, is such thatthe material cannot normally be utilized as a premium motor fuelcomponent. In remote oil and gas producing regions light hydrocarboncompounds are difficult to dispose of profitably. It is desirable insuch situations that the light hydrocarbon intermediates from thewellhead natural gas be utilized profitably instead of being flared orused as refinery fuel. Therefore, what is required is a process by whichthe light hydrocarbon compounds contained within wellhead natural gasmay be converted to a salable gasoline product.

It is an object of the present invention to provide a process for theprofitable utilization of light hydrocarbons found in wellhead naturalgas.

It is a further object of the present invention to provide an integratedseries of processes for the separation of methane and the upgrading ofethane and heavier hydrocarbons into a salable gasoline product.

With these and other objects in mind, the present invention can be morefully understood by referral to the accompanying drawings and followingdescription:

SUMMARY OF THE INVENTION The objects of the present invention areaccomplished by a process for the conversion into gasoline of the lighthydrocarbons contained within wellhead natural gas. The processcomprises fractionation of the wellhead natural gas into a C and lightergas fraction and a C and higher liquid fraction. The C and lighter gasfraction is then separated into a C and lighter gas fraction and a Cliquid fraction. The ethane and propane contained in the C and lightergas fraction is cryogenically separated into a methane rich pipeline gasproduct and a condensed ethane and propane stream. The ethane/propanestream is then pyrolized into olefins and methane with the methane beingseparated therefrom and added to the pipeline gas product. The Cfraction from the debutanizing process is fed to a de-isobutanizing unitto produce a normal butane stream which is sent to gasoline blending andan isobutane stream which is subjected to dehydro- 3,660,272 PatentedMay 2, 1972 genation to form an isobutane and isobutene product mixture.The olefins produced in the pyrolysis step are then reacted withisobutane to produce a lean alkylate product which is then de-butanizedin the de-isobutanization vessel previously described.

Further modifications upon the process for the converslon of lighthydrocarbons contained in the wellhead natural gas may compriseseparating the C and heavier hydrocarbons produced from the pyrolysisprocess and introducing these C and heavier hydrocarbons with thosecontained within the C and heavier liquid fraction obtained from thefractionation process into a catalytic reforming unit to form a C andlighter hydrocarbon fraction, which is recycled to the fractionatingstep for further separation, and a reformate for gasoline blending. Theprocess also may comprise withdrawing a C fraction from thefractionating process, separating isopentanes from the C fraction to beadded to the gasoline inventory while introducing the normal pentanesfrom this fraction into the pyrolysis unit. The ethylene produced fromthe pyrolysis unit may be separated and introduced with a portion of theisobutane extracted from the de-isobutanization process into analkylation unit to produce an alkylated isobutane product which is thenreinjected into the de-isobutanization process. The process may furthercomprise extracting normal butane from the de-isobutanization process,isomerizing the normal butane and reintroducing the isomerized normalbutane into the de-isobutanization process.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be morefully understood by referral to the accompanying drawings in which:

FIG. 1 represents the basic process of the present invention for theconversion of light hydrocarbons contained in a wellhead natural gas topipeline natural gas and gasoline products; and

FIG. 2, consisting of 2a and 211, represents the integrated use ofvarious refinery processes as presented in the present invention for theconversion of hydrocarbons contained in a wellhead natural gas intopipeline natural gas and gasoline products.

DETAILED DESCRIPTION OF THE INVENTION As disclosed, the presentinvention is a process for the production of gasoline from natural gasand is particularly useful in remote areas Where the light hydrocarboncompounds found in natural gas do not have a ready market. Accordingly,in the present invention production of gasoline from natural gas isperformed by con verting and separating the C and higher hydrocarbonscontained in the natural gas to alkylates and reformates which aredesirable as high octane motor fuel components.

The C through C natural gas liquid fraction recovered from natural gasnormally consists essentially of saturated hydrocarbons, for exampleethane, propane, isobutane, and normal butane. Natural gasolinefractions recovered from natural gas typically comprise mostly C to Chydrocarbons and contain substantial quantities of both straight chain,branched chain and ring types. In use as gasoline, it is desirable thatas much as possible of the natural gasoline fraction be in the branchedchain or aromatic form. Accordingly, the natural gasoline fractionrecovered from natural gas is preferably treated first to separateisomers from straight chain hydrocarbons and then to isomerize andreform the straight chain hydrocarbons into the desired branched chainand aromatic hydrocarbons. Therefore, the process of the presentinvention both upgrades the C to C hydrocarbons into longer chainmolecules which may be utilized in high octane gasoline blends and alsoupgrades the C to C hydrocarbons contained within the nautral gas liquidfraction such that a high octane gasoline product is produced with anatural gas pipeline product and no intermediate light hydrocarbons.

In the process for the conversion of light hydrocarbons contained in awellhead natural gas, the initial step of the process comprises thefractionating of the wellhead natural gas into a C and lighterhydrocarbon gas fraction and C and heavier hydrocarbon liquid fraction.In general, a material balance must be calculated for each individualwellhead natural gas stream to be treated so as to determine the amountof refiux heat required, the number of plates in the fractionationcolumn required and the total thermal requirements for the separationdesired. As fractionation columns and related equipment are mechanicaldevices for repeatedly establishing equilibrium between an ascendingvapor and descending liquid for the continuous separation of the twophases, a complete separation of the two phases must be incorporated inany successful design. The number of plates and reflux ratio for afractionation column may be designed so as to give a specific reid vaporpressure gasoline therefrom. In general, the fractionation column willhave from 50 to 200 plates within the distillation tower. The reformatetherefrom containing C and heavier hydrocarbons is blended into afinished gasoline product. The reid vapor pressure of the gasoline maybe adjusted by the addition of sufiicient butane or other low boilingmaterials, for its intended uses.

The debutanizing of the gas fraction from the overhead of thefractionation column is accomplished in a debutanization fractionationcolumn generally having from 25 to 100 plates and using a reflux ratioin the range of 2 to 10. During debutanization, the C hydrocarbons areremoved from the fractionation column and sent to the deisobutanizer,while the overhead constituents from the debutanizer of the C andlighter hydrocarbon gas fraction is subjected to demethanization in acryogenic light ends recovery unit which may consist of but is notrestricted to a molecular sieve packed fractionating column operating atcryogenic temperatures so as to remove the ethane and propanehydrocarbons therefrom and produce a pipeline, methane rich gas product.

The ethane and propane from the demethanization unit are converted topolyolefins in accordance with the present invention. The conversion toan olefinic material is accomplished through a pyrolysis reactionconducted in a conventional pyrolysis unit widely used in commercialprocesses. Pyrolysis processes generally utilize temperatures betweenabout 1400 and 1700 F., approximately atmosphere pressures and reactiontimes between about 0.7 and 1.3 seconds. Partial pressures ofhydrocarbons are preferably maintained at about 10 to p.s.i.a. withsteam generally used as a dilutant to achieve the desired partialpressure. The effluent from the pyrolysis reactor is usually quenched tostop the reaction, water and aromatic based oil are commonly used asquenching media. The degree of conversion of paraffins to olefins is notcritical, but with the practice of the present invention it ispreferably maintained between about 70 and about 90% by weight of thefeed mixture by varying operating conditions in accordance with knowntechniques of operation.

The different products from the pyrolysis reaction include anolefin-rich stream, a tail gas consisting essentially of hydrogen andmethane and a liquid fraction. The tail gas and liquid are separatedfrom the pyrolysis products with the tail gas, essentially a methanerich gas fraction, added to the pipeline product gas stream. Theolefin-rich product stream is reacted in an alkylation unit withisobutane obtained from the deisobutanizing operation.

The alkylation reaction is conducted at a temperature from 30 to 90 F.and at atmospheric pressure. Sulfuric acid catalyst is generallyutilized as the catalyst, although hydrofluoric acid at pressures of-150 p.s.i.g. and temperatures in the range of 70 to 115 F. may beutilized. It is also within the scope of the present invention toutilize phosphoric acid or aluminum chloride as the catalyst or nocatalyst may be utilized for the alkylation reaction, as well as thermalocculation, although the operation conditions change for thesecircumstances. Conventionally, however, the sulfuric acid alkylationprocess is utilized and therefore preferred. A typical product from thealkylation reaction of the olefinic materials with theisobutane-isobutene mixture, for example, from the reaction of ethylenewith isobutane, is 2,2-dimethyl-butane, which has a high octane numberand is an alkylate enhancement for the improvement of the octane numberof the blended gasoline.

The basic process of the present invention may be more fully understoodby referral to FIG. 1. Wellhead natural gas 101 is introduced into afractionating column 102 from which a C and heavier liquid hydrocarbonfraction 103 is produced. A Q; and lighter gas hydrocarbon fraction 104is produced from the upper portion of the fractionating column andintroduced into a debutanizer 105 from which a C through C stream oflight hydrocarbons 106 is produced from the upper portion thereof andintroduced into a cryogenic light end recovery unit 107 from whichmethane 109 is produced as a pipeline gas product stream, and from thelower end of which an ethane and propane fraction 108 is produced whichis introduced into a pyrolysis unit 111, and converted to olefins toform an olefin rich stream 112. This olefin rich stream 112 has themethane 113 stripped therefrom in separator 122. Separated methane isintroduced into the pipeline product gas stream, while the ethylene andheavier olefin stream 114 is introduced into an alkylation unit 115. TheC hydrocarbons from the debutanizer 105 and alkylation product 116 areintroduced into a de-isobutanizing unit 118 from which alkylate 117 isproduced and sent to gasoline blending. Isobutane separated from thede-isobutanizing column 118 is introduced into an isobutanedehydrogenation unit 120 from which hydrogen 123 is vented anddehydrogenated butane stream 121 also containing some isobutane isproduced and introduced into the alkylation unit with the olefinicstream 114 for conversion into alkylate material. Therefore, through theuse of the process of the present invention disclosed in FIG. 1, naturalgas is separated into its methane component for pipeline product gaswith the light hydrocarbon intermediates contained therein upgraded toalkylate materials which are blended with the heavier hydrocarbons forthe production of a high octane gasoline product.

The process of the present invention may involve further upgrading ofthe light hydrocarbon fractions as depicted in FIG. 2. Specifically,wellhead natural gas 201 is introduced into fractionator 202 to form astream of C and lighter hydrocarbons 204 which is then debutanized in adebutanizer 205 to form a C and lighter hydrocarbon stream 206 producedfrom the upper portion thereof which is introduced into a demethanizingcryogenic light ends recovery from which a methane product pipeline gas209 is produced. The ethane and propane rich fraction from the cryogeniclight end recovery 207 forms a stream 208 which is introduced into apyrolysis furnace 211. The particular improvement of the pyrolysisintake stream is derived by extracing from the main fractionator 202 a Cfraction 224 which is introduced into a deisopentanization column 225from which isopentane 226 is produced and sent to gasoline blending,whereas the normal pentane stream 227 produced from the isopentanecolumn 225 is introduced simultaneously with the propane and ethanestream 208 from the demethanizer 207 into the pyrolysis furnace 211. Theolefinic product 212 produced from the pyrolysis furnace 211 is fed toan ethylene separation unit 228 from which an ethylene and methanestream 229 are produced and subjected to further gas separation inseparator 231 of the methane 232 which is fed to the pipeline productgas stream 209. Ethy1= ene effluent 233 from the methane separator 231is then introduced into an ethylene alkylation unit 234 which issupplied with isobutane for the production of alkylate. The alkylateproduct is introduced into a deisobutanizer column 218 with the Cl,stream 210 from the debutanizer 205 to form an alkylate 217 which issent to gasoline blending. Isobutane is removed in stream 219 from theupper portion of the de-isobutanizer column 218 and fed into anisobutane dehydrogenation unit 220 from which hydrogen 223 is producedand an isobutane-isobutene mixture 221 is produced and introduced withthe olefinic material from ethylene separator 218 into an alkylationunit 215. The dehydrogenation may be accomplished by subjecting theparaflins to temperatures between about 200 C. to 600 C. in a reactionzone in the presence of a suitable catalyst. The olefins 230 from theseparation unit 228 are first subjected to purification unit 237 fromwhich pyrolysis naphtha 243 is produced and fed to a catalytic reformer244 from which gasoline 245 is produced. The C and heavier hydrocarbonsseparated from the main fractionator 202 in the form of stream 203 arealso fed to the catalytic reformer 244 from which a hydrogen stream 246,a gasoline stream 245 and a C and lighter gas stream 247 are producedwith the latter material reintroduced into the main fractionator 202.

As mentioned, the olefin stream 238 is introduced with theisobutane-isobutene stream 221 into an alkylation unit 215 from whichalkylate 216 is fed to the de-isobutanizer column 218. A portion ofisobutane stream 219 is extracted in the form of stream 236 and alsointroduced to the ethylene alkylation unit 234 and also introduced asstream 248 into alkylation unit 215. The alkylate stream 216 produced isadded to alkylate stream 235 from the ethylene alkylation unit 234 andintroduced into the deisobutanizer column 218. A further improvement ofthe present process is provided by the removal of normal butane from thede-isobutanizer column 218 in the form of a stream 239 which is splitinto a normal butane stream 240 some of which is added to the gasolineproduct and some of which is fed to a butane isomerization unit 241 forisomerization into an isobutane-normal butane mixture 242 which isreintroduced to the de-isobutanizer column 218. The isomerization may beaccomplished by the use of an aluminum chloride catalyst with thestraightchain material being isomerized and again passed through thecatalyst to remove any unconverted straight-chain material for return tothe isomerization unit. Other catalysts are available for use in theisomerization process, but are well known and need not be furtherdescribed in detail.

In blending finished gasoline it is frequently necessary to adjust thereid vapor pressure. This adjustment is conveniently accomplished byadding butane to the finished products. The butane is added to adjustthe reid vapor pressure of the finished gasoline product to the desiredvalue which will vary according to the intended use of the gasoline asmentioned previously.

Therefore, through utilization of the processes of the presentinvention, a process is provided which is capable of converting lighterhydrocarbons, normally not of valuable use in remote areas, to alkylateand reformate materials which may be blended to high octane gasoline.The natural gas product stream is readily salable. Therefore,utilization of the present invention allows derivation of maximum valuefrom hydrocarbon materials contained within the wellhead natural gas.

While the invention has been described above in respect to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention.

Therefore, I claim:

1. A process for the conversion of light hydrocarbons contained in awellhead natural gas, which comprises:

(a) fractionating the wellhead natural gas into a C and lighterhydrocarbon gas fraction and a C and heavier hydrocarbon liquidfraction;

(b) debutanizing the C and lighter hydrocarbon gas fraction into a C andlighter hydrocarbon gas fraction and a C hydrocarbon fraction;

(c) condensing ethane and propane from the C and lighter gas fraction toproduce a methane rich pipeline gas product;

((1) pyrolyzing the condensed ethane and propane to form olefins andmethane, the methane being separated therefrom and added to the pipeline gas product;

(e) de-isobutanizing the C fraction from step (b) to produce normalbutane stream and an isobutane stream which is subjected todehydrogenation to form an isobutane-isobutene mixture;

(f) subjecting the olefins produced in step (d) and thisobutene-isobutane mixture to alkylation; and

(g) subjecting the alkylate product produced in step (f) to thede-isobutanization of step (e).

2. The process of claim 1 further comprising:

(a) separating the C and heavier hydrocarbons produced in the pyrolysisstep; and

(b) subjecting the C and heavier hydrocarbons from step (a) and the Cand heavier liquid fraction from the fractionating step to catalyticreforming to form a C and lighter fraction which is recycled to thefractionating step and a reformate for gasoline blending.

3. The process of claim 2 further comprising:

(a) withdrawing a C fraction from the fractionating step;

(b) separating the isopentane from the C fraction for addition to thegasoline inventory; and

(c) introducing the normal pen-tane into the pyrolysis step.

4. The process of claim 3 further comprising:

(a) separating the ethylene produced from the pyrolysis step;

(b) extracting a portion of the isobutane produced from thede-isobutanization step;

(c) subjecting the ethylene separated and isobutane extracted toalkylation to form an alkylate product of ethylene and isobutane; and

(d) introducing the ethylene alkylate product into the deisobutanizationstep.

5. The process of claim 4 further comprising:

(a) extracting normal butane from the de-isobutanization step;

(b) isomerizing part of the normal butane to form isobutane; and

(c) reintroducing the resultant isobutane product into thede-isobutanization step.

References Cited UNITED STATES PATENTS 3,060,116 10/1962 Hardin et al.208-93 3,384,571 5/1968 Engel 208-93 3,409,540 11/1968 Gould et a1.208-93 3,502,569 3/ 1970 Hervert 208-93 HERBERT LEVINE, Primary ExaminerU.S. Cl. X.R.

